Reducing the size of waveguide components enables higher levels of integration in planar lightwave circuits (PLCs). Waveguide bends and splitters heavily influence overall component size. Low refractive index contrast waveguides in low index materials such as silica typically have a minimum bend radius of millimeters to several centimeters, which limits device size reduction. In one approach to reduce the size of components, those of ordinary skill in the art use waveguides with high refractive index contrast such that the minimum bend radius is much smaller. Examples for the 1.3 μm and 1.5 μm wavelength regions include core materials with large refractive index such as Si or poly-Si embedded in a low index cladding of SiO2. High refractive index contrast in such systems permits total internal reflection (TIR) to confine light to the waveguide even when the bend radius is reduced to the order of a few microns. Ninety degree bends can be decreased still further in size through the use of resonant cavities or corner mirrors. However, for a given level of interface roughness between the core and clad materials, high refractive index contrast waveguides give rise to more scattering loss than those with low refractive index contrast. Due to scattering loss, PLCs based on high index contrast waveguides may limit device scale integration.
An alternate approach is to use materials with a large refractive index in configurations that permit low refractive index contrast, however, these circuits have weak optical confinement. Examples include SiGe on Si, and SOI ridge waveguides. In such cases vertically etched faces can be used as mirrors to realize sharp waveguide bends. Alternatively, an isolation trench on the outside edge of the bend can be used in conjunction with an offset between the bend waveguide segment and the straight input and output segments to dramatically reduce the bend radius.
Most commercially available PLCs are based on very low loss silica waveguides, which have a low refractive index as well as low refractive index contrast between the core and clad. Moreover, polymer waveguides, which typically have similar low refractive index and low index contrast, continue to receive attention as a potential candidate for PLCs. One proposed solution for silica materials is to etch regions that define a high index contrast bend with tapers on both ends to couple light into and out of the bend region. Overall sizes of such structures are expected to be on the order of 100 μm or greater.